As is well known in the art, conventional computers work with binary digits or bits, which can exist in either a logical state 1 or 0. In a quantum computer, the fundamental unit of information (called a quantum bit or qubit), can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states. In other words, a qubit can exist as a zero, a one, or simultaneously as both 0 and 1, with a numerical coefficient representing the probability for each state. This qubit property arises as a direct consequence of its adherence to the laws of quantum mechanics which differ radically from the laws of classical physics. A physical realization of a qubit is a spin of an electron.
Quantum computation requires qubits which simultaneously satisfy contradictory requirements of both maintaining coherence and yet allowing for easy contact to perform quantum operations. Feynman, R. P., Quantum mechanical computers, Foundations of Physics 16, 507 (1986); Bennett, C. H. & DiVincenzo, D. P., Quantum information and computation, Nature 404, 247 (2000); Nielsen, M. A., Knill, E., & Laflamme, R., Complete quantum teleportation using nuclear magnetic resonance, Nature 396, 52 (1998); and Vandersypen, L. M., et al., Experimental realization of Shor's quantum factoring algorithm using nuclear magnetic resonance, Nature 414, 883 (2001).
It is known that coherence is well preserved in electron spin while operations are best performed on its charge using gates and voltages. Unfortunately, voltage does not couple to electron spin. The quantum operations on a localized electron spin require either nanoscale magnetic fields or complex g-factor engineering combined with spatial manipulation of electron position using voltage controlled surface gates.
Despite tremendous progress in localizing and controlling single electron spin so far no-one has been able to demonstrate a single-qubit operation for qubits based on electron spin.
An alternative solution to the problem of qubit operations on a single spin was proposed by DiVincenzo and collaborators, DiVincenzo, D. P, et al., Universal quantum computation with the exchange interaction, Nature 408, 339 (2000), who suggested quantum computation with exchange interaction. DiVincenzo's basic idea is to replace the two level system based on single-electron spin with selected levels of a composite object consisting of several spins. The manipulation of selected quantum levels proceeds not through operations on single spins but through the manipulation of the coupling J between neighbouring spins due to exchange interaction. For example, two qubit states |0> and |1> for a single total spin S=½ can be identified with two opposite spin directions (Sz) up and down: |0=|↓ and |1=|↑.